It is known to use a measurement scale to determine the relative position of two objects. A readhead mounted on one object reads information from a measurement scale mounted on the other object. In the case of an optically-read measurement scale, the readhead projects light onto the measurement scale, which reflects or transmits the light. The reflected or transmitted light is then detected by the readhead, which can use the detected light to determine the relative position of the scale and the readhead along one or more axes of measurement.
Measurement scales that can be optically read when in combination with a readhead may be referred to as optical encoders. There are two basic types of encoder, namely incremental encoders and absolute encoders.
In the case of an incremental encoder, the measurement scale usually comprises a series of identical markings placed at regular intervals along or around an axis of measurement. A readhead is used to project light onto the scale and to detect the resulting transmitted or reflected light. There are various ways of processing the resulting detection signals to determine position. For example, phase information can be determined from the modification of reflected or transmitted light as the readhead moves along the scale. When the readhead is moved along the direction of measurement, it uses the cyclically changing phase information to calculate relative displacement. Additionally, the phase information can be used to interpolate between periodically repeating scale positions, to achieve a reading that is accurate to within a fraction of the scale period.
In the absence of an additional mechanism for determining absolute position, an incremental encoder can be used to determine only relative displacement. Thus, the measurement scale of an incremental encoder may also have further scale markings in the form of one or more reference marks (indicating a reference position), limit marks may also be included (marks indicating either end of the measurement scale).
In the case of an absolute encoder, the measurement scale has markings that form a series of unique codes, for example codewords, each codeword being associated with a particular position along the scale. It is known to configure the readhead such that it can always read at least one full codeword. The readhead can use a look-up table or algorithm to determine the absolute position on the scale based on the unique codeword, and the system is able to uniquely identify its position on start-up without having to first move to a reference point.
Incremental encoders are often relatively simple and form the mainstay of encoder feedback systems. However, absolute encoders are able to identify uniquely the current position on a scale, wherever that position may be along the length of the scale. Absolute encoders can identify position at power-on without the need to movement and alleviate the need to keep precise count of cyclic signals. Absolute encoders can be important in situations where referencing at power up is not easy, safe or even possible. Also, after a fault condition such as excessive acceleration due to impact or loss of power then the absolute encoder uniquely identifies position without needing relative movement between the scale and read head. The scale must present the read head with enough information to uniquely identify position and this can be done with parallel data lines which form a unique word when read simultaneously, or a long enough section of a serial datastream, or a physically absolute method such as time of flight measurement. However, parallel data presented as a number of serial streams placed side-by-side has the disadvantage of being wide and therefore being sensitive to yaw alignment tolerance. Time of flight techniques require the properties of the light path to be well-known so unless the path is evacuated, temperature and humidity fluctuations and turbulence will affect the measurement.
Usually, an absolute encoder will not have as high a resolution as possible with an incremental encoder due to excessively long codewords needed to uniquely identify position along a useful length of scale, and so it can be desirable to add incremental markings to the absolute encoder in order to be able to interpolate between the positions determined by the codewords. For example, incremental scale markings can be provided on a parallel track to the absolute scale markings. However, the use of such parallel tracks makes the system sensitive to the alignment of the readhead and the scale. Yawing of the readhead can result in errors when combining the absolute and incremental positions.
An alternative approach, in which absolute and incremental scale markings are both formed on a single track, is described in GB 2 395 005 in the name of the present applicant, the content of which is hereby incorporated by reference. The track comprises an amplitude scale consisting of a plurality of reflective and nonreflective stripes arranged parallel to one another and having fixed spacing between the stripes in the measurement direction. Stripes are omitted from the repetitive pattern in order to represent data, in a similar fashion to a linear barcode. Data on the scale are split into words and the read head images enough of the scale to include at least one full word irrespective of the read head position along the scale. Position can be uniquely identified from the word read by the read head. The data words represent an absolute scale, and the repetitive pattern of reflective and nonreflective stripes represents an incremental scale. In this approach, the absolute scale is formed by removing elements of the incremental scale.
It is known that ultrafast laser pulse interaction with a surface can result in the formation of a periodic surface structure, which is generally termed a Laser Induced Periodic Surface Structure (LIPSS) or nanoripple structure. The effect of ‘a regular system of parallel straight lines’ appearing on the surface of various semiconductors damaged by light from a ruby laser was disclosed in “Semiconductor surface damage produced by ruby lasers”, Birnbaum, Milton, Journal of Applied Physics, 1965, Vols. 36, 3688. Since then, these structures have been produced using anything from continuous wave to picosecond lasers, but most commonly femtosecond lasers.
Two groups of LIPSS have been identified, Low Spatial Frequency LIPSS (LSFL) and High Spatial Frequency LIPSS (HSFL), as discussed for example in A. Borowiec, H. K. Haugen, Applied Physics Letters, 2003, Vol. 82, No. 25, pp. 4462-4464, and V. S. Mitko, G. R. B. E. Romer, A. J. Huis in 't Veld, J. S. P Skolski, J. V. Obona, V. Ocelik, J. T. M. De Hosson, Physics Procedia, 2011, Vol. 12, pp. 99-104.
It has been suggested in WO 2009/090324 to use LIPSS structures to represent data, for example for identification, traceability or authentication of objects or documents. In WO 2009/090324, data is represented by the orientation of the LIPSS structures, the orientation being controlled by controlling the polarisation of the laser radiation used to form the structure. The data is read by applying light to the structure and determining the colour of the resulting light received from the structure, with the colour of the light received from the LIPSS structure being dependent on the orientation of the LIPSS structure due to diffraction effects. An image capture device, such as a camera, can be used to capture an image of the surface marked with the LIPSS structures, and the data can be processed to determine the colours that are present and the data values represented by the colours.
The control of the colour of a surface by marking the surface with LIPSS structures has also been described in Ahsan et al, Applied Surface Science, 257 (2011), 7771-7777, 2011; in Dusser et al, Laser Applications in Microelectronics and Optoelectronic Manufacturing VII, Proc. of SPIE, Vol. 7201, 2009; and in Dusser et al, Optics Express 2913, Vol. 18, No. 3, 1 Feb. 2010.
WO 2007/012215 describes the use of nanoripple structures to realise figures, logos, pictures and such-like on the surface of an object, for example on printing rollers which can then be embossed on packaging films. The document also mentions the use of the nanoripple structures to change the physical properties of a surface, for example to improve its adhesive or oil-retaining properties.
US 2009/214885 describes systems and methods for fabricating periodic sub-wavelength nanostructures using laser chemical vapour deposition at or near room temperature.
US 2006/219676 describes a method of making long-range periodic nanostructures inside transparent or semi-transparent dielectrics.
US 2006/028962 describes an optical storage medium having a substrate and a plurality of optically detectable marks imprinted on the substrate, the marks having sub-wavelength width. A polarised light source is used to read the marks.